API: Prefetch customer in order list view (#3768)

Before this change, each order row needed an additional database query just to
resolve the customer id (integer) to customer identifier (string).

With this change, django does a database JOIN.

src/pretix/api/views/order.py

@@ -222,6 +222,8 @@ class OrderViewSetMixin:
             qs = qs.prefetch_related('refunds', 'refunds__payment')
         if 'invoice_address' not in self.request.GET.getlist('exclude'):
             qs = qs.select_related('invoice_address')
+        if 'customer' not in self.request.GET.getlist('exclude'):
+            qs = qs.select_related('customer')
 
         qs = qs.prefetch_related(self._positions_prefetch(self.request))
         return qs